This invention relates generally to adjustable metal-oxide-metal capacitors.
Capacitors are widely used in integrated circuits. One of the most commonly used capacitors is the metal-insulator-metal (MIM) capacitor. A typical MIM capacitor has top and bottom plates and insulation layer in between. The capacitance of a capacitor is proportional to its area and the dielectric constant (k) of the insulation layer, and is inversely proportional to the thickness of the insulation layer. Therefore, to increase the capacitance, it is preferable to increase the area and k value and to reduce the thickness of the insulation layer.
Metal-Oxide-Metal (MOM) capacitor is widely used in precision and high-frequency mixed-signal circuits such as Analog-to-Digital Converter (ADC), Digital-to-Analog Converters (DAC), Programmable Gain Amplifier (PGA), among others. MOM capacitors are less expensive compared to MIM capacitors as MIM capacitors need extra mask layers for fabrication. Also, MOM capacitors can be used for sub-femto farad unit-cells while the minimum size of the MIM capacitor unit-cells is tens of femto farads. MOM capacitors are voltage and temperature independent. Another important feature of MOM capacitor is that their values can be easily programmable by adding/removing MOM capacitor fingers using Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) switches.
FIG. 1 shows an exemplary PGA with an op-amp 5 whose gain is controlled by a MOM capacitor bank 10 and a capacitor 20. FIG. 2 shows an exemplary layout of the capacitor bank 10 with parallel branches each including a capacitor layout 12A in series with a switch layout 14A. The MOM capacitor bank supports programmable capacitor values to adjust gain factors. In conventional method, as shown in FIG. 1, the capacitance of a MOM capacitor bank 10 can either added or removed by closing or opening one or more series switches 14. The series switch 14 introduces following problems, among others:
1) The switch limits the operating bandwidth.
2) The switch can impact the noise content.
3) In high resolution capacitor adjustment, the switch area is a concern and places a restriction on minimum unit cell size.
4) The common-mode voltage affects the switch resistance and linearity of the circuit.